Statistical Characteristics of Electron Pitch Angle Distributions Inside the Magnetopasue Based on MMS Observations

Abstract

Electron pitch angle distributions (PADs) are very important to understand dynamics in the Earth's magnetosphere. Using observations of the Magnetospheric Multiscale (MMS) mission, we statistically investigate the characteristics of several types of electron PADs with energies of 200 eV to 2 keV (low energy) and 2-30 keV (energetic energy) inside the dayside magnetopause (L = 8 ∼ 13). For the low (energetic) energy level, the occurrence rates of pancake, flat-top, butterfly, isotropy, cigar, and rolling-pin distributions are as follows: 50.34% (45.20%), 17.92% (43.41%), 3.07% (7.28%), 4.34% (1.54%), 16.20% (1.20%), and 1.80% (0.06%), respectively. The pancake PAD is mainly located at lower L-shells with a maximum rate near the noonside, but other types of electron PADs occur at higher L-shells. In addition, the occurrence rate of the flat-top PAD is nearly the same for different magnetic local time. The butterfly and rolling-pin PADs occur in the afternoon side. The cigar PAD is mostly located in both the dawnside and duskside but is very scarce in the noonside. Furthermore, the relationships between these PADs and the solar wind dynamic pressure (P dyn) are studied. For low-energy electrons, the occurrence rate of the pancake distribution decreases, and that of the butterfly distribution increases with the enhancement of P dyn. However, for energetic energy level, the occurrence rate of the pancake (butterfly) PAD does not clearly decrease (increase) with the enhancement of P dyn at L ≤ 12. The statistical results reveal the important role of P dyn in electron dynamics inside the magnetopause.

Electron Pitch Angle Distributions in Compressional Pc5 Waves by THEMIS‐A Observations

“…The classification of the ePAD types at the magnetic field troughs of compressional waves is similar to that in previous studies (Li et al., 2020; Liu et al., 2020; Ni et al., 2016). Two indices are defined for PAD identification: γ = G/F and BI = F/C, where G, F, and C are the averaged electron flux at pitch angles ∈ [0°, 25°] & [155°, 180°], [25°, 75°] & [105°, 155°], and [75°, 105°], respectively.…”

“…Since nonwhistler‐type compressional waves tend to occur on the duskside (96%), the drift shell splitting effect (Roederer, 1967; Sibeck et al., 1987) is considered to be a possible scenario. On the dayside magnetosphere with the large L ( L > 7), the electron flux usually has a negative gradient with L (e.g., West et al., 1973), and the electrons often show pancake PADs (e.g., Li et al., 2020). Due to the drift shell splitting effect, the electrons with larger pitch angles on the duskside are from the noonside with higher L‐shells, as shown in the diagram (Figure 3d).…”

Modulation of Whistler Mode Waves by Ultra‐Low Frequency Wave in a Macroscale Magnetic Hole: MMS Observations

“…Furthermore, in the Earth's magnetosphere, the butterfly PAD is very common (H. Li et al., 2020; Ni et al., 2020, 2016), and a statistical study presents that the occurrence rate of electron butterfly PADs is related to the magnetic dips in near‐Earth space (Xiong et al., 2019). However, the wave‐particle interaction related to the formation of butterfly shaped distribution is still controversial.…”

“…Although the electron temperature has primarily negative anisotropy (T ⊥ /T // < 1), it has been confirmed by WHAMP that the unstable butterfly distribution provides the free energy to excite the whistler waves at the minimum magnetic field inside the MH. Furthermore, in the Earth's magnetosphere, the butterfly PAD is very common (H. Li et al, 2020;Ni et al, 2020Ni et al, , 2016, and a statistical study presents that the occurrence rate of electron butterfly PADs is related to the magnetic dips in near-Earth space (Xiong et al, 2019). However, the wave-particle interaction related to the formation of butterfly shaped distribution is still controversial.…”

The Study on the Global Evolution of Energetic Electron Precipitation During Geomagnetic Storm Based on Deep Learning Algorithm

“…Previous studies have indicated that there are two main mechanisms for EEP. In general, the ionospheric isotropic EEP corresponds to the non‐adiabatic tail current scattering (TCS) mechanism due to small field line curvature radius in the nightside (Fu et al., 2021; Shevchenko et al., 2010; Yue et al., 2014), which can violates the first and second adiabatic invariants, and results in electron isotropy distribution (Büchner & Zelenyi, 1989; H. Li et al., 2020; Sergeev et al., 1993, 1988; Yahnin et al., 1997). The electron chaotic motion can cause approximately isotropy PAD and bring about isotropic EEP in pitch angle.…”